As mentioned in the preceding discussion, when a bitmap image is scaled it can suffer losses in quality. If the size of the image is increased, more information must be added to the image. Since the given application (such as Photoshop CS) only has the information in the original bitmap to work from, it must guess in some way as to what information is needed and where the new information should be positioned in the enlarged image. Similarly, if a bitmap is reduced in size, the software must again guess how to reconstruct the pixel array (what to leave in and what to leave out) for the smaller sized image. Re-sampling is the technique or process used to reconstruct the pixel array after the image has been resized.

 

 

This is the image size dialog displayed when you right click on an image’s title bar and choose Image Size from the menu. You will notice in the bottom of the dialog box, a checkbox labelled Resample Image. In the drop list next to this checkbox, there are five re-sampling strategies available. These are the strategies that Photoshop CS can use to reconstruct the pixel array (bitmap) and determine (guess at) any additional information required when the image is resized.

 

Nearest Neighbour	The nearest neighbour technique uses colour values from the closest pixels to interpolate (project) what the colours should be in the scaled image. This sampling technique is fast, (because the algorithm is simple) but it may result in poor quality.
Bilinear	The bilinear technique looks at the average red, green, and blue colour values over a number of pixels. It combines these averages into values to be used in the interpolation of the scaled image. This technique is more involved than Nearest Neighbour, but it provides better quality.
Bicubic	The bicubic technique involves more pixels and more computation than the bilinear technique to determine the information required for scaled images. As a result, this technique may be slower, but it produces a higher quality image.
Bicubic Smoother	This is the bicubic sampling technique with emphasis put on image smoothness. This will also produce results with a higher quality than the Nearest Neighbour or Bilinear techniques.
Bicubic Sharper	This is a bicubic technique with emphasis on image sharpness. Again, this will produce higher quality results than Nearest Neighbour or Bilinear.

 

To use these re-sampling strategies, enter the image size you want in the Image Size dialog box, and then choose the re-sampling strategy you want from the drop list at the bottom of the dialog box. When you are ready, click the OK button to change the image size (changing image size will be discussed further in the next lesson).

 

As an illustration of the image re-sampling strategies, the Eagle image is first scaled up from 424 by 640 pixels, to 1000 by 1509 pixels, using the Nearest Neighbour technique.

 

The following graphic shows the same image scaled with the bicubic re-sampling technique.

 

Can you notice a difference in image quality?